In a digital mobile telecommunication system using TDMA, the information is transferred between radio nodes and one or more mobiles, where the payload information may comprise speech information in a speech connection or data information in a data connection. In FIG. 1, an example is shown of such a system having a radio node BTS1, which communicates with two mobiles MS1 and MS2 which are located within a certain area C1 which is controlled by the node BTS1.
Transmission is performed via allocated radio channels within certain frequency bands, which are divided into two parts; uplink when the mobile sends to the radio node, and downlink when the radio node sends to a mobile.
One example of a digital TDMA-system is GSM, in the standard of which a packet data option is described which introduces packet data in a GSM-network. This technique is called GPRS and involves that data traffic is sent using a packet-switched technique instead of a circuit-switched technique, which is the case for GSM without packet data option. With GPRS present in a GSM-network possible and also the possibility for a user to be connected to the data network all the time.
In a GSM-network having GPRS, the radio channels are used efficiently by that a GPRS-terminal only uses a radio channel when it is sending or receiving data, the rest of the time it is silent and allows other terminals to use the channel. The radio channel PDCH (Packet Data Channel), which is allocated for GPRS, may be of two different types:
Dedicated PDCH, which only can be used for GPRS. The number of dedicated PDCH in each cell is decided by the operator, depending on the load or wish for capacity. This type of channel guarantees that there will always be capacity for GPRS in a cell;
On-demand PDCH, which can be cleared for incoming circuit-switched calls in a loaded cell. In such a channel, circuit-switched traffic always has priority and the GPRS-traffic cannot always be guaranteed.
In FIG. 2, an example is shown of how a set of two radio channels RF1, RF2 can have time slots allocated for both circuit-switched and packet-switched traffic. In RF1, the time slots 2, 3, 4 are allocated for circuit-switched traffic, i.e. speech traffic. In RF2, the time slots 5, 6 are allocated for circuit-switched traffic, while the time slots 1, 2, 3, 4 in RF2 are allocated for packet-switched traffic. In RF1, moreover, two time slots 6, 7 are allocated for On-demand PDCH, i.e. they are used, when needed, for circuit-switched traffic which has executive override, but can also be used for packet-switched traffic if they are free. Finally, the time slots 0, 1 in RF1 are allocated for signalling.
For a terminal to be able to use GPRS, it needs primarily to cedure. Firstly, in order to tell the operator that use will be made of GPRS and, secondly, in order to receive an IP-address to be able to send and receive data. The first step is GPRS-attach, which means that a logical link is formed between the terminal and the SGSN (Serving GPRS Support Node) in question. SGSN is the node which, among other things, performs the transport of incoming and outgoing IP-packets to and from terminals within its so-called SGSN service area, i.e. the area which is taken care of by a SGSN.
The second step is a PDP-Context activation, which means that the specific GGSN (Gateway GPRS Support Node) is informed of that the terminal being there and where it is. GGSN is the node which i.a. takes care of the interface between a GPRS-network and external IP-networks. A PDP-Context activation can be done at any time, e.g. long before data in reality is to be sent, since a user does not use any radio resources as long as he is not sending or receiving data. A PDP-Context activation can remain for any length of time, e.g. several days, all depending on the user's wishes, the limitation of the terminal or any other factor which limits the time.
This activation can be compared to a login on a network, the terminal thereby receiving an IP-address, static or dynamic, which then allows it to start sending and receiving data via IP-packets.
In order then to enable the user to use the data services, having different claims on data rate and delay of data packets offered by the operators, a number of attributes for quality of service have been standardized, as described in the Standard GSM (GSM 02.60, GSM 03.60):                Priority, Service Precedence, is divided into three different classes, indicating the priority of a certain service, and the packets belonging to this service. A service of a lower priority runs the risk of losing data packets when the I which means that a data packet of higher priority has precedence in the network;        Delay is divided into four classes defining which delay that is acceptable in the network;        Reliability is divided into three classes defining the probability for lost data packets, faulty data packets, doublets of data packets and data packets in the wrong order;        Peak Data Rate specifies the highest data rate in the network expected during a whole PDP-Context;        Mean Data Rate specifies the mean data rate expected in the network during a whole PDP-Context.        